Which category do you need? - The main purpose of sunglasses is to cut out light that could otherwise damage your sensitive eyes. In order to inform the customer sunglass lenses are categorised from 0 to 4, 4 are the darkest and cut out the most light, 0 are often transparent (but can have a slight tint), they cut out no light.
For mountain use you should use Category 4 for high mountain ridges in clear conditions or snow and at least Category 3 for valleys with less direct sunlight. Some lenses are photo-chromic, they react to the light conditions and move from Category 3 to Category 4 and vice versa as the conditions dictate. For a fuller discussion of categories:... READ MORE.
Some design features cut out light as well, such as a wrap around styles or removeable side hoods.
Pros and Cons of Lens Material? - there are four materials used to make lenses for sunglasses and they each have their pros and cons, except perhaps for Acrylic which has to many cons to be useful in all but casual use sunglasses (poor durability & clarity with some image distortion) - we don't stock them.
- Glass - has superior optical clarity and scratch resistance but they are heavier and expensive than other materials used for this purpose.
- NXT polyurethane - has superior impact resistance, excellent optical clarity, flexible and lightweight, but they are expensive. Our premium sunglasses are mainly fitted with lenses made from NXT.
- Polycarbonate - has excellent impact resistance, very good optical clarity, is affordable, lightweight and low bulk, although it has less scratch resistance and slightly less optical clarity than glass or NXT.
Lens Colour (Tint)? - The most common lens colour (tint) is Brown, this is because these lenses (along with Grey and Green) are 'colour neutral', that is they cut down on overall brightness without distorting colours thereby accentuating relief.
Features & Construction Methods - Features may not make your sleeping bag warmer per se, but they may make it easier to get in and out of, easier to regulate temperature or protect you against draughts and moisture, all important considerations.
- Polarised Lenses - Glare dimishes clarity of vision & is caused by light bouncing off horizontal surfaces, polarised lenses can eliminate glare, improving visual acuteness...READ MORE..
- Anti-Fog Coating - When you move from cold to warm, or when you heat up one side of your glasses during exertion, they will fog up which is both annoying and dangerous. Anti-Fog coatings reduce this but don't eliminate the effect.
- Anti-Fog Venting - Good venting between the front and back of the sunglass lens can reduce Fogging as the airflow will equalise the temperature that it causing the problem.
- Photochromic Lenses - darken when the sun is brighter to exclude more light, this saves you having extra sunglasses or extra lenses to suit a range of conditions.
- Multiple Lens Options - Some sunglasses come with a range of clip on and clip off lenses of different categories so that you can choose the right lens for the conditions.
- Excluding light from the sides - When sunlight is especially intense, such as at high altitudes, you should wear sunglasses that will stop the light from the sides of your glasses reaching your eyes, you can do this by making sure you buy a wraparound style, by fitting removable side panels or hoods (depending on the model).